Tuned filter circuit



Dec. 5, 1944.

M. R. WINKLER 2,364,260

TUNED FILTER CIRCUIT Filed Feb. 6, 1942 2 Sheets-Sheet l Dec. 5, 1944. R wl 2,364,260

TUNED FILTER CIRCUIT Filed Feb. 6, 1942 2 Sheets-Sheet 2 258mm 33 MM Patented Dec. 5, 1944 UNITED STATES PATENT OFFlC E TUNED FILTER CIRCUIT Marion R. Winkler, Milwaukee, Wis.

Application February 6, 1942, Serial No. 429,791

12 Claims.

The invention relates to improvements in oscillating and tuned filter circuits, and particularly to intermediate frequency or other oscillating radio circuits of normally constant frequency.

It is often customary to provide radio apparatus with an intermediate amplification circuit which has a frequency band of resonance coextensive with that of the audible band width. In the past this circuit usually had a non-uniform response over said band width, with the result that the amplification over the audible range is not uniform.

The present invention relates to the problem of obtaining substantially constant amplification over the desired band width of response of the intermediate frequency circuit.

On the other hand, to obtain high selectivity, it is desirable, that the attenuation at the edges of the resonance band width be as rapid as possible, and this result is also obtained by the present invention.

It, is known that the maximum number of peaks in the resonance curve of a tuned filter network is equal to the number of tuned circuits in said network, andthat these peaks may be made effective by the use of sufficient and proper couplingbetween the different tuned circuits. It is, evident that between any two peaks or maxima there is a valley or minimum.

It is further obvious that the resonance curves.

for the desired band width. To obtain this result,

the maxima of one network or stage of the amplifier are matched with corresponding minima of the other stage. It is furthermore desirable that the attenuation at both edges of the top of the resultant response curve be as rapid as possible, in order to avoid as much interference between adjacent radio frequency stations as possible. The latter result is attained by providing two stages having maxima at their respective band width edges and matching said maxima.

The present invention provides a stage of amplification comprising a transformer employed in an amplifying circuit of the aforementioned type.

The transformer has three or any uneven numi between any two circuits which are not directly adjacent to each other, a condition which results in maximum selectivity or sharpness of tuning.

The invention also provides an intermediate frequency amplifier with two stages, one of which includes a three circuit transformer of the aforementioned type, and another stage having a two circuit transformenall of the transformer circuits having the same resonant frequency, whereby it is possible to obtain an over all amplification which is substantially uniform over the desired band width.

An object of the invention is to provide a transformer having three or any uneven number of resonant circuits, cascaded with another transformer having a number of resonant circuits differing from the first transformer by an uneven number, the two transformers having such response characteristics, that their over all response is substantially uniform over a desired band width of frequencies.

Another object is to provide an amplifier, comprising a three circuit and a two circuit transformer connected in cascade, each transformer having individually, and the combination having collectively great attenuation of the side band frequencies outside of the desired range.

: Another object is to provide an intermediate frequency amplifier affording substantially equal response to all frequencies of a variable band width of frequencies.

Another object is to provide an amplifier with variable coupling.

Another object is to provide a transformer in which the transformer circuits have variable mutual inductance, the adjustment whereof provides for variable coupling.

Another object is to provide a three circuit transformer, having three resonant circuits, whereby the thre circuits have variable mutual inductive coupling and are so arranged, that the coupling between the pairs of adjacent resonant circuits is maintained equal, while said coupling is varied between zero and a maximum, but that the coupling between the first and last resonant circuit is substantially zero at all times and for all adjustments. 1

Other objects and advantages will hereinafter appear.

The accompanying drawings ar illustrative of a two stage intermediate frequency tuning. circuit which incorporates the invention. In the draw-' ings,

Fig. 1 is a side view of a novel arrangement of the transformer of a three circuit intermediate frequency coupling system.

Fig. 2 is a top view thereof, the enclosing casing thereof being shown in section on the line 22 of Fig. 1. r

Fig. 3 is a diagram illustrative of a two stage intermediate frequency amplifier comprising a three circuit stage as shown in Figs. 1 and 2, and

a succeeding two circuit stage, also having variable coupling, while Fig. 4 shows the results obtainable with two modes of adjustment of an amplifier embodying the present invention.

Referring to Fig. 3 the system illustrated therein comprises an electron tube 1 which has among other elements an input grid I, coupled to a high frequency input circuit, a heated cathode l and an anode 1. Variations in the potential of the grid relative to the cathode l produce corresponding variations in the current flowing between the cathode l and the anode l which are connected to a suitable source of energy. The output current of the tube is passed through an inductance 2, which is provided with an electrostatic shield 3, and connected in parallel with the inductance 2 is an adjustable condenser 4. The inductance 2 with the condenser 4 forms an oscillating circuit, the frequency of which can be adjusted by variations of the condenser 4. The inductance 2 may be variably coupled to an inductance 5'which is similar in construction to the inductance 2 and provided with an electrostatic shield 6, and which is connected in series with an adjustable condenser 1, whereby the oscillating circuit formed by the inductance 5 and the condenser 1 may be adjusted to the same frequencies as that of the oscillating circuit of the inductance 2, and the condenser 4. A third inductance 8, provided with an electrostatic shield 9, is connected in parallel with a condenser to form a third oscillating circuit, which can be variably coupled with the second oscillating circuit formed by the inductance and the condenser l. The potential of the third oscillating circuit 8, I0 is impressed upon a grid H" of an electron tube ll, having also a cathode II and an anode II. The potential of the cathodeanode circuit of the tube II, is impressed upon an oscillating circuit comprising the shielded inductance l2, and the condenser l3 which may be adjustably coupled to the inductance 14 of an oscillating circuit, comprising the shielded inductance l4 and an adjustable condenser IS. The potential of the latter oscillating circuit may be impressed upon the grid 16 of an electron tube Hi. It should be observed that the coefiicient of coupling between the oscillating circuit containing the coil 2 and that containing the coil 5, must be equal to that between the latter circuit and that containing the coil 8. Also the Q or ratio of the reactive component to the resistance of the'first named and the last named circuit must be alike. Furthermore the Q of the oscillating circuits containing the coils I2 and M, respectively, must be alike, while the constants of the two amplifying stages are such as to provide for equal band width of their response curves.

The construction and the arrangement relative to each other of the three inductances 2, 5 and 8 is shown in detail in Figs. 1 and 2. The inductance 2 comprises a cylindrical winding 2 which is supported by two insulating side members 2 which side members are arranged for support of the terminals 2 of the coil 2-. Wound about these side members is an electrostatic shield 3, comprising a wire winding, which is pref erably connected in the usual manner to that terminal of the inductance 2 which is nearest to ground. The inductance 2 is connected by means of lead wires 2 to a condenser 4 which is preferably mounted on an insulating plate [1.

The inductances 5 and 8 are similar in construction to inductance 2 aforedescribed, and they are connected to condensers l and I0 respectively, in a similar manner. The inductances 2 and 8 are mounted at equal angles relative to a line connecting their centers and with their axes coplanar with said line, while the inductance 5 is mounted for rotation about an axis perpendicular to its central axis and intersecting the center of the aforementioned line. As indicated in dotted lines in Fig. 2 the inductance 5 can be rotated so that in one extreme position its axis is substantially parallel to that of the two other inductances, in which case there is substantially minimum coupling between it and the adjacent inductances. In the other extreme position, i. e. clockwise rotation of about as shown in dotted lines, the axis of the inductance 5 is substantially at right angle to that of the inductances 2 and 8, in which position there is substantially maximum coupling between it and the adjacent fixed inductances, for the reason that the lines of force pass through the inductances 2 and B substantially parallel with their axes, and form loops which in the dotted position are linked with the rotative inductance so as to produce a relatively close coupling between it and the respective stationary inductance.

It will be further noted that the coupling between the inductances 2 and 8 can be made as small as desired, because few if any of the lines of force interlinked with either inductance are also interlinked with the other inductance, on account of the distance at which they are mounted and on account of the fact that the central axes of the two inductances are parallel to each other, and there is a substantial distance between these axes. The amount of couplin between the adjustable inductance 5 and th stationary inductances 2 and B, respectively, does not affect the value of the inductance 5, so that the resonant frequency of the circuit is not changed by changing the coupling between respective circuits of the tube I and the tube II.

It is obvious, that instead of rotating the inductance 5 as heretofore described, the same may be moved in any other way, provided it has in all positions equal mutual inductance with respect to the other two cooperating coils. This condition is fulfilled, if the relative positions between the inductances 5 and 2, and the inductances 5 and 8, respectively, are maintained alike.

Referring to Fig. 4 the same shows a series of curves, having ordinates drawn on a logarithmic scale, of the relation for a given resonance frequency between gain and frequency for two different conditions of tuning, The curve A lllustrates the response of the second stage, that is, of the two circuit transformer [2-14 of the system shown in Fig. 3, which transformer is constructed in accordance with the foregoing description. The curve B illustrates the characteristics of the transformer of the first stage, having three resonant circuits and resultin in three maxima of its resonance curve as described. The second transformer has two resonant circuits, resulting in two maxima. The two transformers are so proportioned and matched that the reaaeaaco sultant characteristic of the system Fig. 3 is represented by curve C, which has a very flat peak with a relatively constant gain over a relatively wide band width, while the attenuation at the two edges of said band width decreases very sharply and at a much greater rate than that of either the curve A or B alone.

The diagram also illustrates the characteristic of the system adjusted with minimum coupling, that is, for maximum selectivity. In order to facilitate comparison, the two groups of curves of relative gain are drawn so that the maximum amplification at the point of reference is unity for either group of curve.

It will be noted that the curve C, showing the resultant relative amplification of a two stage amplifier of the aforementioned character has a much narrower top or peak, and that the attenuation at both sides of said peak is at a much higher rate than that of curve C. In both cases the selectivity is exceedingly high, though in a circuit corresponding to curve C, the-band width, for any given frequency, is made less than in a circuit corresponding to curve C. It will of course be understood, that this system incorporatin the invention is capable of design for any desired band width.

From the foregoing it will be understood that the number of oscillating circuits of one of the stages may be greater than three with a corresponding increase in the number of peaks of the curve B. In this case the other stage will be provided with a number of oscillating circuits which differs from the number of oscillating circuits of the transformer of the one stage by one or any odd number, so that the response curve for one stage has a number of maxima which differs from the number of maxima of the response curve of the other stage so that intermediate maxima of one match intermediate minima of the other, while the amplitude at the edge of both match so that the resultant response curve has a correspondingly substantially fiat peak of the desired width and the desired rapid attenuation at the edges.

It is essential that the coefficients of coupling between any two pairs of oscillatory circuits of a transformer, which are located symmetrically with respect to the entire series of oscillatory circuits of said transformer, be substantially alike. In other words, in a three-circuit transformer the coefilcient of coupling between the first and the second and that between the second and the third oscillatory circuit must be alike. If a transformer is equipped with five oscillatory circuits, the coefiicients of coupling between the first and second circuit must be equal to the coefficient of couplin between the fourth and fifth oscillatory circuit, and the coefilcient of coupling between the second and the third oscillator circuit must be equal to the coefficient of coupling between the third and the fourth oscillatory circuit. The conditions for transformers with numbers of oscillatory circuits differing from those described are obvious.

The system may be arranged in such a manner that the oscillating frequencies of all of the oscillatory circuits of one or all of the transformers of the system may be varied simultaneously, to permit adjustment of the amplifier for the amplification of different frequencies. However, it is of importance, that under this condition the ratio of reactance to resistance of the respective oscillatory circuits of each transformer conforms to the aforediscussed conditions.

It will be understood that the various tuned oscillating circuits of either stage may be coupled capacitively, or by other equivalent means instead of electromagnetically, as illustrated in the drawings and described in the foregoing.

What I claim as new and desire to secure by Letters Patent is:

1. An amplifying circuit for receiving and amplifying currents within a given band width of frequencies at a substantially uniform ratio, comprising, a, plurality of cascaded amplifying stages, one of said stages comprising a transformer having an uneven number of series coupled oscillatory circuits, and another of said stages comprising another transformer having an even number of series coupled oscillatory circuits, all of said oscillatory circuits having the same natural frequency, the ratio of reactance to resistance of any two oscillatory circuits of either transformer which are located symmetrically with respect to the entire number being alike, and means to provide substantially equal variable onefficients of coupling of any two pairs of adjacent oscillatory circuits of the respective transformer, which are located symmetrically with respect to the entire number, and substantially zero coefficients of coupling between any other two circuits of the respective transformer.

2. An amplifying circuit for receiving and amplifying currents within a given band width of frequencies at a substantially uniform ratio, comprising, a plurality of cascaded amplifying stages,

,one of said stages comprising a transformer havof reactance to resistance, means to provide substantially equal variable coefficients of coupling of any two pairs of adjacent oscillatory circuits of the respective transformers, which are located symmetrically with respect to the entire number, and substantially zero coefficients of coupling between any other two circuits of the respective transformer, and means to vary the reactance of said oscillatory circuits while maintaining substantially equal ratios of reactance to resistance of the respective oscillatory circuits,

3. A circuit for receiving and amplifying all currents within a given band width of frequencies at a substantially uniform ratio, comprising, one amplifying stage including a transformer having three series coupled oscillatory circuits, and another amplifying stage including a transformer having two series coupled oscillatory circuits, all of said oscillatory circuits having the same natural frequency, and the oscillatory circuits of said one transformer being arranged to provide substantially equal variable coefficients of coupling between any two immediately adjacent'circuits, and substantially zero coupling between the other two thereof.

4. A circuit for receiving and amplifying all currents within a given band width of frequencies at a substantially uniform ratio, comprising, one amplifying stage, including a transformer having three series coupled oscillatory circuits, the first and third of said oscillatory circuits having sub stantially equal ratio of reactance to resistance, and another amplifying stage including a transformer having two series coupled oscillatory circuits, all of said oscillatory circuits having the same natural frequency, and the oscillatory circuits of said one transformer being arranged to provide substantially equal variable coefficients of coupling between any two immediately adjacent circuits, and substantially zero coupling between the other two thereof.

5. A circuit for amplifying all currents within a given band width of frequencies at a substantially uniform ratio, comprising, one amplifying stage including a transformer having three series coupled oscillatory circuits, and another amplifying stage including a transformer having two series coupled oscillatory circuits, all of said oscillatory circuits having the same natural frequency and each comprising an inductance and a condenser connected in parallel therewith, the oscillatory circuits of said one transformer being arranged to provide substantially equal variable degrees of mutual inductance between the first and second circuit and between the second and third circuit, and substantially zero mutual inductance between the first and third circuit.

6. A circuit for amplifying all currents within a given band width of frequencies at a substantially uniform ratio, comprising, one amplifying stage including a transformer comprising three series coupled oscillatory circuits, the first and third of said oscillatory circuits having substantially equal ratios of reactance to resistance, and another amplifying stage including a transformer having two series coupled oscillatory circuits and having substantially equal ratios of reactance to resistance, all of said oscillatory circuits having the same natural frequency, and each comprising an inductance and a condenser connected in parallel therewith, the oscillatory circuits of said one transformer being arranged to provide substantially equal variable mutual inductance between any two immediately succeeding circuits, and substantially zero mutual inductance between the other two of said three circuits.

7. A circuit for amplifying all currents within a given band width of frequencies at a substantially uniform ratio, comprising, one amplifying stage including a transformer comprising three series coupled oscillatory circuits the first and third of said oscillatory circuits having substantially equal ratios of reactance to resistance and another amplifying stage including a transformer having two series coupled oscillatory circuits and having substantially equal ratios of reactance to resistance, all of said oscillatory circuits having the same natural frequency, and each comprising an inductance and a condenser connected in parallel therewith, the oscillatory circuits of said one transformer being arranged to provide substantially equal variable mutual inductance between any two immediately succeeding circuits and substantially zero mutual inductance between the other two of said three circuits, and means to simultaneously vary the mutual inductance between any two immediately succeeding circuits by equal amounts.

8. In an alternating current amplifier, a coupling transformer comprising three series coupled oscillatory circuits, each oscillatory circuit comprising an annular inductance coil and a condenser connected in parallel therewith, the central axes of the coils of the first and third circuit being arranged substantially parallel to each other, and means to move the coil of the second circuit relative to the coils of the first and third circuit. while maintaining constant the distance Ill) between the center point of the coil of the second circuit and the center points of the coils of the first and third circuit, respectively.

9. In an alternating current amplifier, a coupling transformer comprising three series coupled oscillatory circuits, each oscillatory circuit comprising an annular inductance coil and a variable condenser connected in parallel therewith, so arranged that the central axes of the first and third coil are parallel with each other and the second coil being rotatable about an axis which is equidistant from the centers of said first and third coil, perpendicular to its central axis and perpendicular to a plane through said first named axes for varying the ratio of mutual inductance between the inductance coils of the first and second and between the inductance coils of the second and third coils respectively, while maintaining the said ratio substantially equal to each other.

10. In an alternating current amplifier, a variable coupling transformer comprising, three series coupled oscillatory circuits, each circuit comprising a unitary annular inductance coil and a condenser connected across the terminals of the respective coil, means for mounting said coils relative to each other so that each turn of the coil of the second of said circuits is at all times in the electromagnetic fields of the coils of the first and third circuits, said means including means to simultaneously vary the mutual inductances between the coils of the first and second and between the coils of the second and third circuits respectively, while maintaining said mutual inductances substantially equal, and simultaneously maintaining the mutual inductance between the coils of the first and third circuit substantially zero.

11. In an alternating current amplifier, a variable coupling transformer comprising, three series coupled oscillatory circuits, each circuit comprising a unitary annular inductance coil and a condenser connected across the terminals of the respective coil, means for mounting said coils relative to each other so that each turn of the coil of the second of said circuits is at all times in the electromagnetic fields of the coils of the first and third of said circuits, said means including means to simultaneously vary in substantially equal degrees the mutual inductances between the coils of the first and second and between the coils of the second and third circuits, respectively, and to maintain substantially zero mutual inductance between the coils of the first and the third circuit.

12. In an alternating current amplifier, a variable coupling transformer comprising, three series coupled cscillatory circuits of equal frequency, each oscillatory circuit comprising, a unitary annular inductance coil and a variable condenser connected across the terminals of the respective coil, means for mounting said coils relative to each other so that each turn of the coil of the second circuit is at all times in the electromagnetic fields of the coils of the first and third circuit and to provide variable but at all times substantially equal degrees of mutual inductance between the coil of the second circuit and the coils of the first and third circuit respectively, and substantially zero mutual inductance between the coils of the first and third circuit.

MARION R. WINKLER. 

